P2518

A/C Refrigerant Press Sensor B Circ High

Indicator

The PCM has detected a voltage reading from a particular a/c refrigerant pressure sensor that does not fall within acceptable limitations. Consult the vehicle service manual for the location of this particular sensor/switch.

Code Set Parameters

Pressure fluctuations in the air conditioning system are received as voltage variations by the PCM. Variations in system voltage that are either too low or too high will cause a trouble code to be stored and a service engine soon lamp to be illuminated.

Symptoms

The most distinguishable symptom may be that the air conditioning system is inoperative. Other symptoms could include a stored code and an illuminated malfunction indicator lamp. The air conditioning system may also be disabled and the "ON" indicator lamp (usually in the a/c switch) may also flash repeatedly.

Common Causes

The most common cause of this code being stored is due to a low freon condition in the air conditioning system. Other causes can include excessive system pressure, a faulty a/c refrigerant pressure switch (there are typically between 1 and four pressure sensors in a system, depending upon manufacturer), faulty electrical connectors, damaged, shorted, or open wiring, or a faulty climate control head.

Common Misdiagnosis

Technicians report that systems are often overfilled with freon when this code is presented, resulting in catastrophic failure. Please do not add freon to the a/c system without first verifying that it is necessary. Only qualified personnel (certified to handle freon under federal mandate), using the proper tools and equipment should attempt to service your a/c system. Single hose charging kits (with no gauge or an inadequate gauge) sold in auto parts and department stores are often a recipe for disaster.

Diagnosis

The average automotive air conditioning system utilizes between one and four refrigerant pressure switches to provide safe and efficient operation

Freon is used to distribute oil throughout the air conditioning system, so you can see that refrigerant pressure is crucial to a/c system longevity and operation

Pressure switches can be threaded onto metal a/c lines or inserted into the compressor

These switches are usually of a pressure sensitive "ON/OFF" design

As pressure rises in the a/c system it presses against spring tension in the low pressure switch

When sufficient pressure is reached for proper system operation, contacts inside of the switch are closed and the compressor clutch electrical circuit is completed, allowing the clutch to engage

High pressure switches operate in a similar manner

When system pressure becomes excessive, it presses against spring tension inside the switch

When system pressure reaches a level that could be detrimental to the compressor and other major components, contacts inside the switch are opened and the electrical circuit for the a/c compressor is opened causing the a/c clutch to disengage

Repeated failures (usually 3) of this nature are recorded by the PCM

After a predetermined number of failure cycles are completed, a code is stored and a service engine soon lamp is illuminated

Some models also disable the a/c compressor clutch until the code has been cleared.
Several tools will be instrumental in successfully diagnosing this code

A suitable OBD-II scanner (or code reader), a digital volt/ohmmeter, and a set of air conditioning gauges (or an a/c recycling machine with integrated gauges) will be most helpful in performing a successful diagnosis

Always retest the system after repairs are completed to ensure success.
If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data

This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored

Continue by clearing the code and operating the vehicle to see if it returns

This will help to determine whether or not the malfunction is intermittent

After the codes are cleared, test drive the vehicle to see if the code returns

If the code fails to immediately return, you may have an intermittent condition

Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made

Low Pressure
Connect the scanner or code reader and record all stored trouble codes and freeze frame data

Clear codes, connect the a/c gauges, start the engine and turn the a/c on high

Test for voltage and ground signals at the low pressure switch electrical connector

Compare your actual findings with manufacturer's specifications and repair open or shorted circuits as needed

If voltage and ground signals coincide with manufacturer's specs, test the low pressure switch resistance using the digital volt/ohmmeter

Compare your findings to manufacturer's specifications and replace the switch as required.
High Pressure
Connect the scanner or code reader and record all stored trouble codes and freeze frame data

Clear codes, connect the a/c gauges, start the engine and turn the a/c on high

Observe the pressure gauge readings

If the high pressure gauge reading exceeds the manufacturer's maximum specification (usually 275 to 300 psi), turn the a/c system off and allow pressure to decrease

Turn the a/c back on and observe the electric condenser fan (if so equipped) operation

If the electric cooling/condenser fan/s are not operating, diagnose the cause of this problem

Lack of electric cooling/condenser fan operation will cause high pressure to soar

If the fan is operating and a/c system pressure exceeds maximum recommended specs, suspect a restriction in the orifice, expansion valve, accumulator, receiver dryer, or condenser.
If the high pressure reading is within specs and the compressor clutch fails to engage due to a high pressure condition, perform a resistance test on the high pressure switch and compare your findings with manufacturer's specifications

Replace the high pressure switch as required

If the switch does fall within acceptable specifications, check continuity in the system circuit

Repair or replace faulty electrical wiring and connectors as needed.
If a/c refrigerant pressure switches and circuitry fail to yield the cause of this malfunction, suspect a faulty climate control head or a/c compressor switch

Test the unit according to manufacturer's specs and replace components as necessary.